In an experiment on almond trees conducted in California, researchers found that a lack of bees and other wild insects to pollinate crop plants can reduce harvest yields more drastically than a lack of fertilizer or a failure to provide the crops with sufficient water.

When crops are adequately pollinated, on the other hand, the plants bear more fruit and their nutrient content changes.

Together with students and colleagues at the University of California, Berkeley, Professor Dr Alexandra-Maria at the Albert-Ludwigs-Universitat Freiburg manipulated almond trees by preventing bees from pollinating blossoms with the help of cages, allowing the bees to pollinate the blossoms, or pollinating them by hand.

In addition, the researchers watered and fertilized trees in accordance with local practices or gave them only little water or no fertilizer.

In the case of several almond trees, they combined the various manipulations in order to study in isolation and in combination the effects on harvest yield and the composition of nutrients in the nuts.

The almond trees that were pollinated by hand produced the most nuts, but they were also very small. By contrast, a tree that was left unpollinated hardly produced any nuts at all - but the few that it did produce were very large.

The yield of the trees pollinated by bees was roughly 200 percent higher than that of self-pollinated trees.

Fertilization and watering only had an effect on harvest yield in combination with the pollination manipulations. However, the inadequately watered trees lost more leaves, and the leaves of the unfertilized trees increasingly turned yellow.

This led the scientists to the conclusion that an almond tree can compensate for a lack of nutrients and water in the short term by directing stored nutrients and water to the fruits but cannot compensate for insufficient pollination.

Furthermore, the scientists demonstrated that the composition of nutrients differs depending on the pollination mode: Nuts from the self-pollinated trees contained a lower proportion of linoleic acid but a higher proportion of vitamin E.